1. Field of the Invention
The present invention relates to a preparation process of a degradable copolymer of aliphatic polyester useful for medical materials and substitutes of general purpose resins.
More specifically, the invention relates to a process for preparing a degradable compolymer of aliphatic polyester compounds which is obtained by reacting two or more homopolymers of the aliphatic polyester compounds in the presence of a catalyst in a reaction mixture containing an organic solvent; and a degradable copolymer of aliphatic polyester prepared by said preparation process.
The degradable copolymer of aliphatic polyester which can be prepared by the process of the invention is very excellent, as compared with homopolymers, in heat resistance, transparency, elongation, flexibility, plasticity and shaping ability, and can be processed into medical materials and substitutes for general purpose resins in the form of a molded article, film, sheet, filament, yarn and foam having strength, elongation, transparency and heat resistance which satisfy the object.
2. Description of Related Art
The polymer of aliphatic polyester is characterized by hydrolyzing with ease in the presence of water and waste articles can degrade without contaminating the environment when the polymer is used as a general purpose resin. Consequently, the polymer has already received attention as an environmentally friendly substitute for general purpose resins prior to the application of the invention. Further, in the case of using the polymer in a living body as a medical material, the polymer can be decomposed and absorbed in the living body after accomplishing the object without giving toxicity to the living body. Thus, the polymer has received attention before the present application as a medical material which is friendly for the living body.
In many cases, however, an aliphatic polyester homopolymer alone cannot fully satisfy the desired properties as a medical material or a substitute for general purpose resins.
For example, polylactic acid is brittle, hard and lacking in flexibility. These properties have led to problems when the polymer is processed into films, filaments and molded articles. That is, conventional technology could not provide with ease aliphatic polyester having softness, flexibility, transparency and high heat resistance.
U.S. Pat. Nos. 3,736,646 and 3,982,543 have disclosed the use of a volatile solvent as a plasticizer in order to provide flexibility for a lactide copolymer. However, when the polymer is processed into film or molded articles and used for preservation of products, food containers or medical devices, gradual evaporation of the solvent eliminates the plasticizing effect and causes problems on the quality retention of the products. At the same time, the released solvent from the polymer is possible to give a toxic effect on the food or the living body which is in contact with the polymer and has problems in view of safety insurance.
U.S. Pat. Nos. 3,636,956 and 3,797,499 have disclosed an L-lactide/D,L-lactide copolymer which can be used for a surgical suture. The patent has also described that plasticizers such as glycerol triacetate, ethyl benzoate and diethyl phthalate are added, in order to provide plasticity, to the copolymer which lacks plasticity. When no plasticizer is added to the copolymer, it is a problem that the copolymer has a low elongation at break of only about 10%. On the other hand, when these plasticizers are added to the copolymer, it is naturally anticipated that food packaging containers and medical devices eluate these plasticizers upon use and give a toxic effect on the living body, or the plasticizers eluated from abandoned articles contaminate the natural environment.
Polylactic acid and other polymers having a high brittleness, high hardness and low flexibility have been mixed with other polymers having a high ductility or plasticity, high softness and high flexibility to improve their physical properties. However, compatibility between homopolymers of different kind is generally low. Thus, it is extremely difficult to obtain a polymer alloy having improved properties by mixing two or more homopolymers of different kind in the absence of a compatibilizer. For example, when the compatibility between homopolymers is low even though two homopolymers individually have transparency, it is difficult to obtain the polymer alloy having improved properties while maintaining transparency.
U.S. Pat. No. 4,057,537 has disclosed a non-rubber-like copolymer having impact resistance, brittleness resistance and heat resistance which is obtained by heating an L-lactide/.epsilon.-caprolactone mixture containing 75 to 90% of L-lactide at a temperature between the melting point of L-lactide and 200.degree. C. in the presence of a catalyst. The patent has also described that a copolymer containing 15% or more of .epsilon.-caprolactone is soft and transparent. These copolymers, however, have disadvantages of low or no crystallinity and low heat resistance.
Japanese Laid-Open Patent HEI 7-173266 has disclosed a preparation process of a linear lactic acid type copolyester by subjecting lactide, that is, the cyclic dimer of lactic acid and linear aliphatic polyester consisting of an aliphatic dicarboxylic acid ingredient and an aliphatic diol ingredient to a ring-opening polymerization and ester exchange reaction in the presence of a ring-opening polymerization catalyst.